Last January I bought a Duffy 18 electric boat. Shortly after, on an evening cruise in Channel Islands Harbor, my then girlfriend wondered why of the hundreds of electric boats in the harbor none had solar panels. Why indeed? It seemed to me to be a natural combination. I set out to find the answer.
The first stop was Central Coast Electric Boats, the local Duffy dealer. My thought was that surely Duffy must have tried to solarize their boats. And that was the case according to Frank Laza, owner of Central Coast. He said that the Duffy factory had experimented with solar at some point in the past but had abandoned the project. He said they found that it was impractical. So much for being able to simply buy a solar kit from Duffy.
In further conversations with Frank about Duffy’s solar project, it became clear that a couple of conditions likely doomed their project. The first was that at the time of their experiments the available solar equipment was not as efficient as that available today. The second had to do with the goal of their attempt. I surmised that if they set as a goal continuous operation the size of the array of panels would greatly exceed the available space on the boat.
But what if the goal was merely to replace the charge used rather than to be able to operate the boat non-stop? I knew that my usage consisted of at most an hour to an hour and a half daily and not always every day. This goal would significantly reduce the needed capacity of the system.
How to quantify my actual usage? After several unsuccessful attempts, including plugging the boat into an amp meter after what I thought was an average use and watching the draw hourly, I thought about giving up.
Chance intervened. I happened to mention my query to my friend Rob Tryon who in the past had worked in the solar industry. He had two words for me: battery monitor. He explained how a battery monitor can measure the amp-hour usage, the parameter that must be known in order to be able to properly size the system. What’s more, he recommended a model made by Victron that could not only measure amp-hour consumption but could also record a history and calculate average amp-hour usage. Bingo!
The installation was fairly straight forward: mount the indicator on the instrument panel, install the shunt, a low-resistance resistor in series with the negative battery lead and connect them. It was immediately apparent that the monitor provided far more useful information than just amp-hour usage. It showed the amp draw, battery voltage, hours of operation remaining, charge cycles as well as several other very useful parameters.
Several weeks of simply using the boat followed. After approximately 25 charge cycles the average usage centered on 50 amp-hours. I now had the datum I needed to size the array. This model Duffy uses a 36 volt battery; therefore I needed a charge controller able to provide that voltage to charge it. I also had to take into account the hours of sunlight. At 6 hours of sunlight, I calculated I needed a little over 8 amps per hour to replace the average draw. Eight amps times the charge voltage of ~42 volts gave me around 350 watts. I knew that over the course of a day, the sunlight falling on the panels varies because of angle of incidence, clouds, etc. and that a constant rate per hour could not be counted on. I wanted around 500 watts in order to provide for the varying rate as well as system losses inherent in any system such as this.
Searching for the controllers, I found one manufactured by Genasun that provided the right output. In addition, it would accept an input voltage that would allow two standard 12 volt (17 volt nominal) panels to be connected in series. This meant I could reduce the number of controllers from 4 to 2. When I mentioned this to Rob, he not only said he could get the controllers but also that he was upgrading the panels on his boat and would sell me the old ones. Score! The only drawback was that his panels were 100 watt units, but at the price he quoted I couldn’t pass it up.
So there you have it. I finished installing the second pair of panels and controller this last Monday. Monday night I “burned” 55 amp-hours using the boat. At 5:30 yesterday afternoon the battery was fully charged. Last night we used 72 amp-hours, traveling 5 ¼ nautical miles in an hour and a half at full throttle with the stereo blasting. I checked the state of charge 45 minutes ago with the sun directly overhead (max charge conditions) and the system was outputting 8 amps and the battery was 35% charged. It will be at 100% around 5:30-6:00 tonight.